As a wafer for a vertical silicon device such as an IGBT (Insulated Gate Bipolar Transistor), a wafer having an N conductivity type manufactured by an FZ method (Floating Zone Method) is generally used. Since the IGBT is a device that uses a wafer in the vertical direction, it is affected by quality of a bulk of the wafer. Therefore, the FZ method that can readily obtain a wafer having less defects is adopted.
However, increasing a diameter of the wafer by the FZ method is difficult, and this method is not suitable for mass production. Therefore, there have been suggested a nitrogen-doped wafer having a defect region that is an N region and low oxygen concentration (Patent Literature 1), and an RTA-processed wafer having an N region and low oxygen concentration (Patent Literature 2), which are manufactured by a CZ method (Czochralski Method).
It is to be noted that the defect region of the wafer in the CZ method varies in greatly dependent on a pulling rate of a silicon single crystal ingot. Grown-in defects, which can be considered as voids corresponding to agglomerations of holes that are point defects called “vacancies” are densely present in a substantially entire region in a crystal radial direction in an area where the pulling rate is high, and the region where these defects are present is called a V region. Further, when the pulling rate is reduced, an OSF ring produced in a crystal peripheral portion shrinks toward the inside of a crystal, and it is eventually annihilated. When the pulling rate is further reduced, an N (Neutral) region where excess and deficiency of the vacancies and interstitial silicon are low appears.
Since a width of the pulling rate required for obtaining an N-region silicon single crystal is narrow and a yield ratio is poor, a high-cost wafer is provided, but it has almost no defect in its crystal, and hence it is used as an IGBT wafer.
If a variation in resistivity is considerable in a wafer radial direction and in the vertical direction with respect to the IGBT wafer, a difference in resistivity is produced between devices, which can be a cause of damage.
Therefore, as the IGBT wafer, a wafer having a radial resistivity distribution which is not greater than 5% has been also suggested (Patent Literature 3, Patent Literature 4).